1. Field of the Invention
The present invention relates to an FSK signal receiving device designed to receive and demodulate an FSK signal in a communication apparatus used in, e.g., a mobile radio communication system and an optical communication system.
2. Description of the Related Art
For example, in a mobile radio communication system, the FSK (Frequency SHIFT Keying) system is widely used as a modulation system for digital signals.
According to a conventional FSK signal receiving device, an FSK signal received through an antenna is amplified by a high frequency amplifier, and the amplified signal is input to two mixers. In these mixers, the received FSK signals are respectively mixed with a local oscillation signal generated by a local oscillator and a .pi./2-shifted local oscillation signal obtained by shifting the phase of the local oscillation signal by .pi./2 using a phase shifter, thereby performing frequency conversion. Since the frequency of the local oscillation signal is set to be substantially equal to the carrier frequency of the received FSK signal, base band signals are directly output from the mixers. The FSK demodulated signals output from the mixers are respectively input to low pass filters to remove their high-frequency components. Thereafter, the signals are respectively amplified by low frequency amplifiers and amplitude-limited by amplitude limiters. The resulting signals are input to a detector.
If the above-described conventional receiving device receives an FSK signal having a carrier frequency fc and a frequency shift .+-..delta. corresponding to the binary value represented by a digital signal, since a frequency fL of a local oscillation signal is set beforehand to be equal to the carrier frequency fc, and local oscillation signals to be supplied to the mixers have a phase difference .pi./2, two base band signals having frequencies, each of which is equal to the frequency shift .delta., and also having orthogonal phases can be obtained from the low pass filters. These base band signals can be expressed as follows: EQU a(t)cos(2.pi..delta.t) EQU a(t)cos(2.pi..delta.t+.pi./2)=-a(t)sin(2.pi..delta.t)
where a(t) is the amplitude determined by an input voltage to the receiving device, the gain of the high frequency amplifier, and the characteristics of the mixers and the low pass filters.
The base band signals output from the low pass filters are respectively shaped by the low frequency amplifiers and the amplitude limiters to be supplied to the detector. In the detector, the signal level of one of the base band signals is sampled in synchronization with an edge of the other base band signal to determine the phase relationship between the two base band signals, and the polarity of the frequency shift .delta. is discriminated on the basis of the phase relationship. Assume that the signal level of a first base band signal is detected at the leading edge of a second base band signal, and the exclusive OR between the detected first base band signal and the second base band signal is "0". In this case, it is determined that the second base band signal is delayed with respect to the first base band signal. For example, it is discriminated on the basis of this phase relationship that the frequency shift .delta. is positive. In contrast to this, if the exclusive OR between the detected first base band signal and the second base band signal is "1", it is determined that the second base band signal is advanced with respect to the first base band signal. It is then discriminated on the basis of this phase relationship that the frequency shift .delta. is negative.
That is, this receiving device is designed to discriminate whether the frequency shift .delta. is positive or negative by utilizing zero-crossing points (leading or trailing edges) of two base band signals whose phases are shifted from each other by .pi./2.
However, in the above-described FSK signal receiving device, since zero-crossing points of two base band signals are utilized to discriminate whether the frequency shift .delta. is positive or negative, discrimination timings are limited to the positions of zero-crossing points of the two base bands, and discrimination cannot be performed at other positions. If, therefore, the data transmission rate is high as compared with the shifted frequency, especially when one slot of data corresponds to 1/2 the shifted frequency or less, a given data slot transits to the next slot before the next zero-crossing points of the first and second base band signals come. As a result, changes in codes of reception data cannot be detected sometimes. That is, data transmitted at a high transmission rate cannot be completely received sometimes, thus limiting the transmission rate of data.
In addition, in order to handle data transmitted at a high transmission rate, the frequency shift .delta. must be set to be large as compared with a modulation wave frequency fm (a frequency at which the frequency shift of a received FSK signal is switched between +.delta. and -.delta. and which corresponds to 1/2 the data transmission rate). This, however, requires a wide transmission band.